This invention relates to an optical magnetic recording member which is characterized by an enhanced Kerr effect.
Recently, attention has focused on an optical magnetic recording medium which is capable of high density recording, for use in a system wherein recording and reproducing are carried out by a laser ray. In the reproducing process, when a polarized laser ray is either reflected at, or transmitted through, the surface of a magnetic recording medium, the polarization plane of the laser ray is rotated accordingly. It is desirable that this rotation angle (Kerr rotation angle) be as large as possible.
Certain materials have been used for the purpose of enhancing the Kerr rotation angle. The widely accepted view hitherto is that, in order for a material to have an enhancing effect on the Kerr rotation angle, it must be optically transparent. Thus, transparent dielectric materials such as SiO, SiO.sub.2, ZnS, Si.sub.3 N.sub.4, AlN, TiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.3 or the like, have been provided on a surface of a magnetic layer, that is, a magnetic recording medium. It has been also known that, among materials used for increasing the Kerr rotation angle, those with a relatively large index of refraction have a relatively larger enhancing effect on the Kerr rotation angle. Especially in the case where reproducing on a magnetic recording member is carried out by irradiating a laser ray from the side of the transparent substrate of said magnetic recording member, it is desirable that the material used for enhancing the Kerr rotation angle have a higher index of refraction than in the case where reproducing is carried out by irradiating the substrate from the side adjacent to air, because the refractive index of glass or PMMA used for the substrate is about 1.5.
The wavelength of the laser used in the reproducing process with an optical magnetic recording member is about 800 nm, so that it is sufficient that the material used for increasing the Kerr rotation angle be transparent to the laser wavelength of about 800 nm. It is not always necessary that such material be transparent over the whole range of visible radiations, as are the conventional transparent dielectric materials mentioned above. From this perspective, the inventors have examined various materials, and as a result, have found that a very large enhancing effect on the Kerr rotation angle can be obtained by using, instead of the conventional dielectric layer, a material which has an index of refraction as high as 3.0-3.8, and also has a small absorption coefficient for a wavelength of 800 nm, and, in addition, has an index of refraction so much higher than that of glass or PMMA as to be suitable also for use in the case where reproducing on a magnetic recording member is effected by irradiating a laser ray from the side of the magnetic layer that is adjacent to the transparent substrate.